Ventriculomegaly, also called “hydrocephalus,” is defined as increased volume of cerebrospinal fluid within the cerebral ventricles. This is manifested by dilation of some or all of the cerebral ventricles, most often the lateral ventricles. Ventriculomegaly may result from a variety of causes, including genetic syndromes, congenital anomalies of the brain and spinal cord, and in utero infection or exposure to teratogens. Ventriculomegaly is frequently associated with other fetal abnormalities, often involving intracranial structures or the spine.

The prognosis for a fetus with ventriculomegaly is related to the degree and severity of associated abnormalities, as well as to the degree of ventricular dilation and the thickness of the cerebral cortex. In some cases of ventriculomegaly, magnetic resonance imaging (MRI) may provide additional information about the fetal brain that cannot be identified with ultrasound, such as polymicrogyria (excess folding of the brain surface into abnormally small gyri), periventricular heterotopia (clumps of neurons that failed to migrate to the brain surface but rather remained along the ventricular wall), or other neuronal migrational abnormalities.

Ventriculomegaly is diagnosed sonographically when there is abnormal dilation of the cerebral ventricles. From 18 weeks gestation onward, ventriculomegaly is diagnosed when the width of the lateral ventricle at the atrium (located at the posterior tip of the choroid plexus on axial view) measures more than 10 mm (Figure 6.1.1) and the choroid plexus dangles from its medial attachment toward the lateral wall of the ventricle (Figure 6.1.2). It is important to note that a small amount of fluid between the medial wall of the lateral ventricle and the choroid plexus can be a normal finding, especially between 18 and 20 weeks gestation. Before 18 weeks, the diagnosis of ventriculomegaly is based on the appearance of the lateral ventricle and dangling choroid plexus, because abnormal ventricular dilation may be present with a ventricular width less than 10 mm (Figure 6.1.3).

When ventriculomegaly is diagnosed based on dilation of the lateral ventricles, it is important to assess the third and fourth ventricles for dilation. A dilated third ventricle is diagnosed when there is abnormal separation of the walls of the third ventricle by cerebrospinal fluid (Figure 6.1.4). Increased fluid in the fourth ventricle is best seen on axial views of the posterior fossa.

Because ventriculomegaly is commonly associated with other anomalies of the central nervous system, the fetal cranium and posterior fossa should be evaluated carefully when the diagnosis is made. The cranium should be assessed, looking for a lemon shape that would suggest a meningomyelocele or looking for a defect in the cranium through which tissue is herniated, which would indicate an encephalocele. The posterior fossa should be assessed for evidence of a Dandy–Walker malformation or the Chiari II malformation associated with meningomyelocele (Figure 6.1.5). Irregularity of the inner wall of the lateral ventricles may indicate periventricular heterotopia (Figure 6.1.6).

Aqueductal stenosis is obstruction of the cerebral ventricular system at the aqueduct of Sylvius, which lies between the third and fourth ventricles. Aqueductal stenosis leads to hydrocephalus, with dilation of the lateral and third ventricles due to obstruction of the flow of cerebral spinal fluid from the third ventricle to the fourth. The fourth ventricle and posterior fossa are typically normal with this anomaly. Aqueductal stenosis is sometimes the result of an X-linked recessive genetic trait and, therefore, is more commonly found in males than in females. Other causes of aqueductal stenosis include in utero infections, such as toxoplasmosis, cytomegalovirus (CMV), and syphilis, or teratogen exposure.

With aqueductal stenosis, both lateral ventricles and the third ventricle are dilated, while the posterior fossa and fourth ventricle are normal (Figures 6.2.1 and 6.2.2).

Dandy–Walker malformation is characterized by a posterior fossa cyst that communicates with the fourth ventricle, agenesis or hypoplasia of the cerebellar vermis, and dilated lateral and third ventricles. The posterior fossa cyst, or Dandy–Walker cyst, is a fluid collection that extends from the fourth ventricle between the cerebellar hemispheres to the cisterna magna, splaying the cerebellar hemispheres apart. The cerebellar hemispheres are thus separated by the fluid of the Dandy–Walker cyst, and the cerebellar hemispheres are often abnormally formed or hypoplastic. The degree of ventriculomegaly is variable.

Dandy–Walker malformations are associated with various genetic syndromes and chromosomal abnormalities. They can also result from in utero infection.

When a Dandy–Walker malformation is present, fluid is seen between the cerebellar hemispheres and the cerebellar vermis is absent or hypoplastic (Figure 6.3.1). The Dandy–Walker anomaly often has a characteristic keyhole shape between the cerebellar hemispheres (Figure 6.3.2). The posterior aspect of the cyst may be quite large, and the cerebellar hemispheres are sometimes flattened (Figure 6.3.3) rather than having their normally rounded shape. Ventriculomegaly (Figure 6.3.3) often accompanies the posterior fossa abnormalities.

Anencephaly is a neural tube defect involving the fetal head, characterized by absence of the cranium. Dystrophic brain tissue may form in the empty shell behind and above the face, but this tissue tends to atrophy as pregnancy progresses. The face, from the orbits down, is usually normally formed. Anencephaly constitutes approximately 45% of all neural tube defects, making it the most common neural tube defect. It occurs in approximately 1/1,000 pregnancies and is more prevalent in patients with Irish, Scottish, and British ancestry than other populations.

This severe anomaly is often diagnosed during the latter part of the first trimester. Those cases not identified in the first trimester are typically detected in the second trimester, a common time for performance of a fetal anatomic survey.

Polyhydramnios is commonly present, likely because fetal swallowing is impaired. The prognosis is extremely poor, with virtually all fetuses dying at birth.

Absence of the fetal cranium is the hallmark of anencephaly. Although dystrophic brain tissue is commonly seen as amorphous tissue protruding from the skull base and floating above the face during the first trimester, by the second trimester, the dystrophic brain tissue in the expected region of the cranial vault typically has regressed and retracted into the skull base. As a result, the sonographic findings in the second trimester include absence of the brain and skull; no bony forehead or tissue above the orbits and lower face; and no head above the cervical spine posteriorly. On a coronal image of the fetal face, the nose, mouth, and orbits are visible, but the forehead is absent above the orbits. On longitudinal views of the spine, no head will be seen superior to the spine (Figure 6.4.1). In some cases, depending on fetal position, transvaginal imaging is useful for improved visualization of the region of the head and face (Figure 6.4.2). Polyhydramnios may be present.

Encephalocele is characterized by a defect in the cranium through which intracranial contents herniate outside the skull. Occipital encephaloceles are a form of neural tube defect, comprising approximately 5% of neural tube defects. Like other neural tube defects, maternal serum α-fetoprotein levels are usually elevated with encephaloceles. Occasionally, the defect is closed, covered by the scalp or skin, in which case the maternal serum α-fetoprotein level is likely to be normal.

The majority of encephaloceles are located in the midline, most commonly posterior involving the occiput, less commonly parietal or frontal. The herniated encephalocele sac may contain dystrophic brain tissue or it may contain only meninges and fluid. Within the head, ventriculomegaly may be present, and in some cases the “lemon” sign, characterized by flattening or concavity of the frontal bones, may be seen during the second trimester. Cysts of the scalp or cranial bony lesions may mimic an encephalocele, but these lesions do not communicate with intracranial contents.

Encephalocele is a feature of Meckel–Gruber syndrome, an autosomal recessive genetic abnormality. The other findings with Meckel–Gruber syndrome include polycystic kidneys, polydactyly, cleft palate, cardiac anomalies, and liver cysts. Severe oligohydramnios is often present with Meckel–Gruber syndrome, due to absent or decreased urine production from the polycystic kidneys.

Encephaloceles can also result from amniotic band syndrome. In such cases, they may involve any part of the skull and are often not in the midline.

The prognosis for encephaloceles detected in utero is usually guarded, but depends on the location of the encephalocele, the amount of brain tissue herniated into the encephalocele sac, and the presence and severity of associated findings.

The cranial defect of an encephalocele appears as an interruption in the bony skull with intracranial contents herniating through the defect (Figure 6.5.1). The herniated sac is usually rounded and contains tissue as well as fluid.

With an anterior encephalocele, a soft-tissue mass will be seen protruding anteriorly through a defect in the frontal bones between the orbits (Figure 6.5.2). Hypertelorism, or widening of the normal distance between the orbits, may be present. On a sagittal midline view of the fetal face, the encephalocele may be seen as a protrusion of soft tissue between the forehead and the tip of the nose.

Encephaloceles seen with Meckel–Gruber syndrome are usually occipital. Visualization may be difficult because of severe oligohydramnios, a common finding because of the associated renal abnormalities. With Meckel–Gruber, the fetal kidneys will be enlarged and are either abnormally echogenic or filled with multiple cysts (Figure 6.5.3). Dandy–Walker malformation may also be seen with this syndrome.

Encephaloceles resulting from disruption with amniotic bands are usually asymmetric and off midline (Figure 6.5.4).

Scalp or cranial osseous lesions should not be mistaken for encephaloceles. Careful assess of the underlying scalp, bone, and meninges will confirm that the lesion is extracranial and does not communicate with intracranial contents (Figure 6.5.5).

The corpus callosum is a band of neural tissues that connects the right and left cerebral hemispheres. Agenesis of the corpus callosum results from failure of formation of part or all of the corpus callosum. Because the anterior portion of the corpus callosum develops before the posterior portion, partial agenesis of the corpus callosum typically affects the posterior aspect.

Agenesis of the corpus callosum is often associated with intracranial and extracranial anomalies, as well as various syndromes and chromosomal abnormalities. Other central nervous system anomalies are present in about 85% of cases, including such anomalies as Dandy–Walker malformation and interhemispheric cysts. Agenesis of the corpus callosum is a hallmark of Aicardi syndrome, a rare X-linked condition with very poor prognosis, found in females only because affected male fetuses are thought to miscarry early in pregnancy. Smooth brain syndromes (lissencephaly) are often associated with agenesis of the corpus callosum. When truly isolated, agenesis of the corpus callosum has a prognosis that can vary from near-normal development to severe handicaps.

The most common sonographic finding with agenesis of the corpus callosum is an abnormal configuration of the lateral ventricles. Instead of their normal convergence anteriorly, the lateral ventricles are aligned parallel to the falx with the frontal horns displaced laterally. The occipital horns are typically disproportionately dilated compared to the frontal horns, giving the ventricle a teardrop shape, a finding termed colpocephaly (Figure 6.6.1). This latter finding is not usually evident until after 20 weeks gestation. In addition to changes in the lateral ventricles, the third ventricle is elevated and often mildly dilated (Figure 6.6.2). A sagittal midline image of fetal head will show sulci radiating from the roof of the third ventricle, secondary to absence of the corpus callosum and cingulate gyrus (Figure 6.6.3). In addition, the cavum septum pellucidum is absent (Figure 6.6.4). Because the cavum
septum pellucidum may be present with partial agenesis of the corpus callosum, the diagnosis of partial agenesis is often not made prenatally.